The field of the invention includes laminated, organic plastic film which is used for packaging. More particularly, the present invention relates to laminated, anti-static film for use in skin-packaging operations in which static-sensitive items are packaged.
The conventional skin-packaging process is widely used in the electronics industry for packaging items such as electronic modules or circuit card assemblies which are to be stored or transported. The skin-packaging procedure consists of heating a plastic film to a semi-molten state, drawing a vacuum between the film, the item to be packaged, and a coated substrate such as fiberboard and then drawing the film over and around the item, forming a heat sealable bond with the coated substrate. As a result, the vacuum between the film, the packaged item and the substrate is thereby maintained.
During the packaging process friction between the film and the packaged item may cause a buildup of static electricity which can discharge and damage static-sensitive components in the item or the item itself. Moreover, it is also possible that, while the item is contained in the package, a static discharge may occur between the packaged item and another object outside of the package. It is therefore necessary to provide protection against static discharge for static-sensitive items which are undergoing a skin-packaging operation.
Anti-static storage packages are known in the art. One example of such a package is found in U.S. Pat. No. 2,327,713 to Hunter, which discloses an anti-static packaging material made up of two outer layers of nonabrasive paper which encloses an opaque high density paper. The material is useful for protecting photographic film from static discharge and exposure to light during storage. Another example of a static-reducing storage material is found in U.S. Pat. No. 3,572,499 to Mondano, in which a laminated packaging material is disclosed which consists of a layer of metal foil which is sandwiched between two sheets of electrically conductive plastic. The material taught in the Mondano patent can be used to form a sealed package by forming a thermal bond between the plastic laminations of two opposing sheets of material. However, because of the presence of the inner layer of metal foil the Mondano material is not useful for skin-packaging operations. Still another laminated plastic structure is taught in U.S. Pat. No. 3,143,364 to Klein, wherein an aluminized polyethylene sheet is bonded to a non-porous substrate. The Klein patent process is intended primarily to bond a film of polyethylene to another material having a nonporous surface such as nonmetallized polyethylene, and is intended primarily for sealing together plastic pipe parts; the process is inappropriate for use in skin-packaging operations.
Other laminated plastic materials utilized to form food containers are disclosed in U.S. Pat. Nos. 4,309,466 and 4,284,672 both to Stillman. In the Stillman patents, the disclosed packaging materials are both composed of layers of plastic which are selected for their properties of strength and durability, and not for electrical characteristics. Hence, they provide no protection for static-sensitive items.
In U.S. Pat. No. 4,363,071 to Rzepecki et al., a multi-layer static-dissipative mat is disclosed in which a layer of conductive material is enclosed between two layers of resistant material so that static charges can be collected by the conductive material and conducted to ground through one of the resistive layers. However, the materials used to construct the layers of the mat are selected for strength and durability and are unsuitable for use in skin-packaging operation. Other U.S. Patents which provide examples of packaging materials with electrically conductive or resistive characteristics are: U.S. Pat. Nos. 2,341,360; 3,839,135; 4,208,696; and 4,301,040. However, none of the examples pertain to skin-packaging operations and they are therefore inappropriate for use in such operations.
Other references containing information which is germane to the packaging or plastic material arts are: U.S. Pat. Nos. 3,085,025; 2,248,579; and 2,274,712.
Therefore, there exists a need for a film material useful in skin-packaging operations which possesses the ability to prevent the buildup of an electrostatic potential between itself and a static-sensitive item during packaging operations and while the item is contained in the package.
It is therefore the principle object of the present invention to provide a material useful in skin-packaging operations which prevents the buildup of electrostatic potential between itself and a static-sensitive item which is to be packaged.
It is a further object to provide such a material which also reduces the likelihood of an electrical discharge occurring between the packaged item and another object while the item is contained in the package.
These and other objects of the invention will become more apparent from the ensuing description when taken together with the drawing.